An electrostatic chuck is used to attract and hold a processing object such as a semiconductor wafer, a glass substrate, etc., inside a plasma processing chamber that performs etching, CVD (Chemical Vapor Deposition), sputtering, ion implantation, ashing, etc.
The electrostatic chuck is made by interposing an electrode between a ceramic base material such as alumina, etc., and by sintering. The electrostatic chuck applies electrical power for electrostatic attraction to the built-in electrode and attracts and holds the substrate such as the silicon wafer, etc., by an electrostatic force. A wafer processing apparatus includes such an electrostatic chuck.
In recent years, the plasma output in etching apparatuses that use plasma has been increasing. As the plasma output increases, the temperature of the wafer increases; and wafer temperature fluctuation is one cause of the decrease of process yields.
Conventionally, plasma has been used to clean the interior of the chamber regularly to remove residue and products adhered to the chamber inner surfaces. At this time, so-called waferless plasma cleaning may be performed in which processing of the surface of the electrostatic chuck is performed without covering the surface with a dummy wafer. When cleaning using waferless plasma cleaning, the surface of the electrostatic chuck is exposed directly to the cleaning plasma such as O2 gas, CF4 gas, etc.
Under such conditions, it is desirable for the electrostatic chuck to have plasma resistance, a high insulation breakdown voltage, and a long life.
However, for example, among electrostatic chucks, the chucking force is generated only on the electrode in a Coulomb-type electrostatic chuck. Accordingly, by providing the electrode at the lower portion of a sealing ring mounted at the outermost perimeter of the electrostatic chuck surface, the chucking force of the sealing ring portion increases; and highly efficient wafer cooling is possible. It is desirable for the electrode to have a configuration that is nearly a perfect circle to generate a uniform chucking force at the sealing ring portion and provide a uniform wafer temperature. For example, in the case where the configuration of the electrode is an ellipse, the electrode surface area of the minor axis portion of the ellipse is narrower than the electrode surface area of the major axis portion of the ellipse; therefore, the chucking force at the minor axis portion is lower than the chucking force at the major axis portion; the chucking force for attracting and holding the wafer is nonuniform in the surface; and the wafer cannot be cooled uniformly. Therefore, it is desirable for the electrode to be disposed uniformly up to the vicinity of the outer perimeter of the ceramic dielectric substrate. If the electrode is disposed uniformly up to the vicinity of the outer perimeter of the ceramic dielectric substrate, a uniform chucking force can be obtained in a wide area of the wafer; and the temperature distribution of the wafer can be set to be uniform. However, in the case where the electrode is disposed up to the vicinity of the outer perimeter of the ceramic dielectric substrate, the insulating distance between the electrode of the ceramic dielectric substrate and the wafer which is the chucking object becomes short. Therefore, for example, when the configuration of the electrode is an ellipse, the insulating distance between the wafer and the electrode on the major axis side is shorter than the insulating distance between the wafer and the electrode on the minor axis side; therefore, there is a risk that the insulation breakdown voltage of the electrostatic chuck may undesirably decrease.
In JP-A 2003-504871 (Kohyo), a configuration is discussed in which an electrode extends on the outer side of a groove of a cooling gas in a Johnsen-Rahbek electrostatic chuck. However, because the electrode is provided in the interior of the ceramic dielectric substrate, it is difficult to easily and accurately detect the position of the electrode from outside the ceramic dielectric substrate. For example, it is necessary to perform the measurement using an ultrasonic flaw detector, etc., to ascertain the position of the electrode provided in the interior of the ceramic dielectric substrate; but the measurement accuracy of the ultrasonic flaw detector is, for example, about 0.5 millimeters (mm). Therefore, it is difficult to identify dimensions less than 0.5 mm when measuring using the ultrasonic flaw detector.
Also, because the position of the electrode after the sintering of the ceramic dielectric substrate is different according to the conditions when sintering such as the electrode outer diameter, the shrinkage factor of the ceramic dielectric substrate, etc., the distance from the outer perimeter of the ceramic dielectric substrate to the electrode in the interior after the sintering fluctuates easily. Therefore, when performing grinding of the outer perimeter of the ceramic dielectric substrate, if the grinding is performed undesirably up to a position proximal to the electrode, locations undesirably occur where the distance is short from the outer perimeter of the ceramic dielectric substrate to the outer perimeter of the electrode. Thereby, a problem occurs in that the risk of dielectric breakdown is high.
Thus, an extremely difficult operation is necessary to accurately ascertain the position of the electrode of the interior of the ceramic dielectric substrate, perform grinding of the outer perimeter of the ceramic dielectric substrate, and cause the outer perimeter of the electrode and the outer perimeter of the ceramic dielectric substrate to be as proximal as possible. Therefore, in a conventional electrostatic chuck, a sufficiently ample margin is provided in the distance between the outer perimeter of the electrode and the outer perimeter of the ceramic dielectric substrate to reduce the risk of dielectric breakdown. However, as the distance between the outer perimeter of the electrode and the outer perimeter of the dielectric substrate is increased, the chucking force that is generated at the sealing ring of the electrostatic chuck outer perimeter portion undesirably decreases; and the temperature of the chucked wafer undesirably increases. Further, when the fluctuation of the distance between the outer perimeter of the electrode and the outer perimeter of the dielectric substrate is large, a problem occurs in that a portion of the wafer outer perimeter portion or the entire circumference of the wafer outer perimeter portion cannot be cooled uniformly.
In JP-A 2012-235037 (Kokai), a configuration is discussed in which a built-in electrode overlaps under an outermost perimeter sealing ring in a Coulomb-type electrostatic chuck cross-sectional view. However, the chucking force for attracting and holding the wafer is generated in the portion directly above the sealing ring overlapping the electrode. Therefore, one important component to provide a uniform wafer temperature distribution is to dispose the outer diameter of the electrode in the region where the electrode overlaps the sealing ring to be proximal to the outer perimeter of the electrostatic chuck, and to make the built-in electrode and the electrode outer diameter even more uniform.
In JP-A 2009-302346 (Kokai), a configuration is discussed in which the sealing ring width is widened to provide a configuration in which the built-in electrode overlaps under the outermost perimeter sealing ring. However, the plasma inside the process also erodes the ceramic dielectric. Therefore, the sealing ring surface which is a direct contact portion with the wafer may be eroded by the plasma; and the surface state of the sealing ring portion may fluctuate. Then, the chucking force at the sealing ring portion decreases; the wafer temperature distribution becomes nonuniform; the wafer temperature changes partway through the process, etc.; and the fluctuation of the surface state of the sealing ring portion undesirably causes the life of the electrostatic chuck to decrease.